1. Field of the Invention
The present invention relates generally to devices for measuring the dusting properties of a substance, and more particularly to devices for measuring the dusting properties of pigments and dyes.
2. Description of the Related Art
In the field of pigments and dyes, there are increasing attempts being made to produce dust-free and non-dusting preparations, which do not emit dust while being processed. Such non-dusting preparations are particularly desirable so as to avoid color contamination when various products are handled, and they also make accurate apportioning possible. Dust losses are avoided, and because it is cleaner and safer to work with such preparations, the health risk involved in working with toxic pigments and dyes is also reduced.
The safest procedure is to use paste preparations, adding enough of such additives as surface-active substances and solvents as to attain a non-dusting, pasty state. The agents required for a particular application, such as dispersing agents, anti-settling agents and binders, are often also incorporated at the same time. In such pastes, the pigment concentration depends on the fineness and texture of the given pigment as well as on the amount of binder it requires and may be between approximately 30 and 90%. In producing the pigment pastes, the starting material is dried pigment, which is mixed with the various components.
The disadvantage of the pigment pastes is that they are difficult to apportion, and they necessitate careful cleaning of the packing drum.
Quite a number of methods are already known. According to German patent application DE-OS No. 27 23 921, the tendency of metal chromates to produce dust is suppressed by adding a combination of phthalic and terephthalic acid esters and esters of fatty acids, along with surface-active substances, to an aqueous suspension of pigment and drying it after filtration. Such preparations produce less dust, but they are not permanently non-dusting. A product produced according to Example 1 of the above patent application yielded a dust production value of 83 mg/100 g, as measured with the Heubach dust-production measuring device. Further methods and agents are disclosed in German patent application DE-OS No. 29 31 771 and DE-OS No. 25 23 096, U.S. Pat. No. 3,560,134, European patent application EU-OS No. 0056 160 and German patent application DE-OS No. 28 41 566.
The parent application, U.S. patent application Ser. No. 763,437, filed Aug. 1, 1985, discloses and claims permanently non-dusting pigment and dye preparations, which in addition to from 75 to 97.5% pigment contain a surface-active substance in an amount from 0.5 to 10% and from 2 to 25% of an agent which when intensive shear stress is applied causes the dried, homogeneous mixture of pigment or dye and surface-active substance to reach the smear point, which preparations are at the smear point as defined herein. Any surface-active agent suitable for the particular pigment or dye type used and for the intended later application can be used as the surface-active substance; exeamples include ionic or nonionic agents available on the market. However, a long-chain polyester of the "highly effective dispersing agent" type is particularly preferred, in particular a polyester derived from a saturated or unsaturated aliphatic .omega.-hydroxycarboxylic acid with at least 4 carbon atoms between the hydroxy group and the carboxy group and a total of at least 9 carbon atoms including the carboxy group, or from a mixture of a hydroxycarboxylic acid of this kind and a carboxylic acid lacking hydroxy groups, in particular one having up to 20 carbon atoms.
Among these suitable polyesters are those described in German patent application DE-OS No. 21 62 484, which are hereby expressly incorporated by reference.
What is essential here is that the surface-active substance is not applied to the dry pigment, nor has the pigment already been dried; instead, the still-moist pigment, as it comes from the production process, is treated with the surface-active substance, because otherwise the desired surface properties are not obtained. Therefore the agent is suitably also mixed with the pigment suspension and only then is it dried. The term "pigment" also includes dyes.
Any substance that, after being introduced into the dry mixture of pigment and surface-active substance that is to be attained, causes the mixture to attain the smear point under intensive shear stress can be used, and the various technical requirements of the fields of application, such as enamels, paints, printing inks and plastic dyes, can be taken into consideration at the same time. Among these are water, organic solvents, plasticizers and substances which become liquid in response to elevated temperatures, such as waxes. Useful agents here are those which either do not volatilize, or do not do so significantly. For reasons of economy, mineral oil is generally preferred, because it brings about excellent results and is relatively inexpensive.
As the mineral oil, aliphatic, alicyclic and/or aromatic hydrocarbons which are flowable at room temperature and the boiling point of which is above 70.degree. C. are particularly useful. Aliphatic hydrocarbons which may contain various quantities of cycloaliphatic and/or aromatic hydrocarbons are preferred. This category thus includes not only technical grade mineral oils made from variously refined crude oils but also synthetic hydrocarbons, such as Fischer-Tropsch mixtures with the given boiling ranges. Accordingly, this includes all unsaponifiable substances of arbitrary origin, mainly comprising hydrocarbons, obtained for example from crude oil, froms tars or the distillation products thereof, or from low-temperature carbonization.
Special preference is given to synthetic isoparaffins. However, water is also very suitable, and in particular from 10 to 15% of water can be added when small quantities of surface-active agent are employed. In the case of preparations made non-dusting by the addition of water, the end product should then be packaged in watertight or vapor-tight containers, so that no significant amounts of water can escape.
These permanently non-dusting pigment preparations are produced by adding 0.5 to 10% by weight, relative to the total weight of the end product, of a suitable surface-active agent to the filter suspension of the pigment; after homogeneous distribution, the combination of pigment and surface-active agent, which may optionally have been isolated in the form of a press cake, is dried, and the dried product is then mixed with an agent which brings the mixture to the smear point when intensive shear stresses are applied, this agent being in particular mineral oil having a boiling range from 70.degree. to 360.degree. C., preferably 180.degree. to 280.degree. C., and especially 180.degree. to 250.degree. C.; intensive shear stress is applied until the smear point is reached, and then the product is granulated in a manner known per se if desired.
If water is to be used as the substance that brings the mixture to the smear point, this water can be added subsequently, or else the mixture of pigment or dye and surface-active agent, the latter perhaps in the form of a press cake, is not fully dried, so that a residual water content of 2 to 25%, and in particular 10 to 15%, remains, and the moist mixture thus obtained is brought to the smear point by the application of shear stress and, if desired, granulated. This water content is then present in the end product as well, aside from slight changes occurring during further processing.
The homogeneous distribution of the surface-active substance in the filter suspension of the pigment is achieved by performing thorough mixing in a known manner. Isolation as a press cake is achieved by means of suitable filtration devices, such as filter presses, drum filters, suction filters, etc., which yield a press cake or an equivalent adequately water-free product which can then be dried in a technically appropriate manner.
Treatment with the substance which brings the mixture to the smear point is carried out in such mixing apparatus as a kneader mixer, intensive mixer, paddle mixer or any other suitable device which generates sufficient shear stress and brings the product to the smear point within no longer than approximately 2 hours. Preference is given to degrees of shear stress by which the smear point is attained within 15 to 45 minutes, and in particular within about half an hour. In the process, the product usually appears in the form of a flowable fine granules which is practically non-dusting.
Within the meaning employed in this patent application, the smear point is reached when a cohesive, putty-like mass, as defined for example by determining the oil adsorption according to DIN 53199 (see also Ullmanns Encyklopaedie der technischen Chemie [Ullman's Encyclopedia of Industrial Chemistry], 4th edition, Volume 18, page 565), has not yet formed. The smear point as defined in this patent application is reached when the above-mentioned flowable fine granulate, which is non-dusting, has formed and produces a slight smearing effect on the walls of the mixing devices. Hence the amount of dust-binding agent, such as oil, required to achieve this smear point is less than the amount of oil required to determine the oil adsorption according to DIN 53199 and is shown in Henry A. Gardner and George C. Sward, "Paints, Varnishes, Lacquers and Colors", 12th Edition, March, 1962, distributed By Gardner Laboratory, Inc., as Stage A and 2 bottom pictures on page 245, FIG. 443 and is about or shortly before the "wet point" as shown on pages 251- 253.
By adding the surface-active substance to the pigment suspension before drying is carried out, optimum wetting is achieved, which makes it possible to obtain a virtually absolutely non-dusting pigment preparation by adding mineral oil, for example, to the dried product comprising pigment and surface-active substance. Only relatively small quantities of mineral oil are needed. When equal amounts of surface-active substance and mineral oil are added to the previously dried product, almost twice as much mineral oil is needed, e.g., more than 20%, instead of 10% as in the method according to the present invention, to achieve the same degree of absence of dust production.
This method can be applied in principle to all pigments and dyes which are in the wet state and are separated out as a filter suspension. It can be used equally well for inorganic and organic pigments and dyes such as lead chromate, lead molybdate, zinc chromate, mixed-phase pigments such as nickel-titanium yellow, chrome-titanium yellow, etc., phthalocyanines, anthraquinones, azo pigments, lake-type azo pigments, disazo condensation pigments, isoindolines, indigold dyes, quinachridones, and perylenes, among others.
The mixture of pigment or dye suspension, which usually contains about 50% water, and surface-active agent is first thoroughly homogenized and then either dried directly or, if it is desired that the water content be reduced further, it is further dewatered in a suitable press, for instance a high-pressure press, and only then taken to the drying stage, thereby saving drying costs. In this way it is possible to obtain press cakes having a solids content of up to 85%. This mixture can be dried in the usual manner, for instance on belt or suspended-belt dryers with an ambient temperature of 120.degree. to 130.degree. C., or in vacuum dryers where temperatures of 100.degree. to 120.degree. C. are normally used. Since the mixture contains organic material, care should be taken to insure that the drying conditions, in particular the temperature, do not cause any damage to the material, for instance by local overheating.
The present dust production measuring device was developed in order to be able to perform extremely accurate analyses of the dust-producing tendencies of powdered and granular substances, because quantitative information about dust-producing behavior is important for the sake of safe industrial processes and operations. The tendency toward dust production was heretofore judged by analogy with the proportion of fines in the starting material, or by estimation, but such methods can give rise to considerable errors, because dust particles are frequently generated only by some type of movement and by abrasion; in contrast, the tendency to produce dust during materials handling is ascertained herein in a manner that very closely approximates actual conditions. To this end, the test material remains in motion during the entire period of measurement.